Diffusion through vessel walls

Four appendixes are included. Appendix A contains standard materials selection used by many refiners and contractors in petroleum processing equipment. Appendix B contains a rules of thumb overview of refinery materials of construction. Appendix C contains background information on hydrogen diffusion through vessel walls, and Appendix D contains a standard specification for steel line pipe. 

In another case, acidic water was used to clean the inside of the water Jacket that surrounded a glass-lined vessel. Some hydrogen diffused through the wall of the vessel and developed sufficient pressure to crack the glass lining. 

This would represent a reversible oxidation. The reaction with H2 could then be [M-02]it -f 112( 7) —> [M()2H]w H(gf). Studies of the exchange of H2 with D2 at temperatures near 600°C [G. Boati et al., Nuovo Cimento, 10, 993 (1953)] show that atoms are initiated at the walls of a silica vessel by O2 diffusing through the walls of the flask ... 

The formation of a platelet aggregate requires the recruitment of additional platelets from the blood stream to the injured vessel wall. This process is executed through a variety of diffusible mediators which act through G-protein-coupled receptors. The main mediators involved in this process are adenosine diphosphate (ADP), thromboxane A2 (TXA2), and thrombin (factor Ila). These mediators of the second phase of platelet activation are formed in different ways. While ADP is secreted from platelets by exocytosis, the release of TXA2 follows its new formation in activated platelets. Thrombin can be formed on the surface of activated platelets (see Fig. 2). 

Table III gives the half-life of radicals, for the same range of variables as are presented in Tables I and II. In an experiment with flash photolysis, the radical concentration may be about 1 mm. and the half-life is about 1 iMsec. In an experiment with a intense steady source close to the reaction vessel, the radicals may reach a concentration of lO Ycm. and the half-life is about 0.01 sec. In an experiment with weak sources or light sent through a typical monochromator, the radical concentration is typically 10 /cm. and the lifetime is 1 to 10 sec. In the latter case radicals will have time to diffuse to the walls, and thus two investigators may unwittingly be studying two totally different reactions even when they wanted to study the same photolysis, if one had a weak light source and wall reactions and the other had a strong light source with short-lived radicals.

The particular way in which the walls of the blood vessels in the central nervous system are constructed results in their being impermeable to many substances, thereby limiting the ability of molecules to pass from the blood into the brain. This phenomenon is called the blood-brain barrier. Molecules may cross the blood-brain barrier by mechanisms of active transport, or by being sufficiently lipid soluble that they can diffuse through the hydrophobic core of the lipid membranes that form the boundaries of the cells composing the blood-brain barrier. Most psychoactive drugs are sufficiently lipid soluble that they can pass from the blood into the brain by passive diffusion. 

The amount of hydrogen (substance A is hydrogen) which diffuses through the vessel wall is equal to the reduction in the amount stored ... 

Fig. 14.2. Double-capsule technique designed to hold an experimental system at a fixed, known oxidation state. This arrangement is held in a pressure vessel containing a fluid at high temperature and pressure. The pressure is transmitted to the experimental system by the flexible walls of the gold and platinum tubes, and at the experimental temperature hydrogen, but not other components, is able to diffuse through the metals.

In the previous section, the permeation of solutes through uniform lipid membranes was discussed however, cell membranes and cellular barriers are not perfectly uniform (Figure 5.1). Proteins interrupt the continuous lipid membrane and provide an additional pathway for the diffusion of water-soluble molecules. Protein channels in the membrane, for example, permit the selective diffusion of certain ions. In the blood vessel wall, water-filled spaces between the adjacent endothelial cells provide an alternate path for transport. 

Rapid and facile generation of capsules from tandem assembly in aqueous media is amenable to encapsulation of water-soluble compounds. Encapsulation of ICG dye within PAH/H2PO4 aggregates was shown by Yu et al. Enzyme encapsulation and the feasibility of capsules to serve as reaction vessels was demonstrated by Rana et al. In their study, they encapsulated acid phosphatase enzyme in PLL-citrate-silica sols and suspended the spheres in a solution containing fluorescein diphosphate. Fluorescence increased in intensity within the shell walls as fluorescein was formed by enzymatic cleavage of phosphate groups. This study showed that microcapsules could serve as reaction vessels that allow enzymatic action to take place in a protective environment and allow for reactants and/or products to diffuse through permeable shell walls. 

Consider a steel reservoir containing an acidic electrolyte and the outside of the reservoir is exposed to the atmosphere. Hydrogen will be evolved inside the walls of the vessel because of corrosion. A part of the atomic hydrogen will diffuse through... 

It has been argued that the large values of k for the reactions of NO with oxymyoglobin and oxyhemoglobin would severely limit the amount of free NO in blood. However, further work indicates that this reaction is limited by the rate of NO diffusion through the membrane of red blood cells and that there is a region near the vessel wall in flowing blood that is fiee of red blood cells. ... 

The loss of trace analytes can be minimized by conducting all operations in a system that is almost completely sealed hermetically from the ambient atmosphere, and by using vessel materials characterized by a small effective. surface area [3J. Surfaces should be free of fissures and preconditioned as necessary to minimize physical adsorption of the analyte (e.g., through ion exchange or hydro-phobic interactions). It is also important to consider the possibility of diffusion into or through the vessel walls. 

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